In accordance with enlargement of the market of portable electronic apparatus such as portable phones, notebook-type personal computers, and digital cameras, a secondary battery having a large energy density and having a long lifetime is expected as a cordless power source of these electronic apparatus.
Then, in order to meet such a demand, a secondary battery having alkali metal ions such as lithium ions as a charge carrier and using the electrochemical reaction accompanying the donation and receipt of the electric charge thereof is being developed. In particular, a lithium ion secondary battery having a large energy density is currently widely prevalent.
Among the constituent elements of a secondary battery, an electrode active substance is a substance that directly contributes to battery electrode reaction which includes charging reaction and discharging reaction, and hence plays a central role of the secondary battery. In other words, the battery electrode reaction is a reaction that is generated accompanying giving and receiving of the electrons by application of a voltage to the electrode active substance that is electrically connected to the electrode disposed in the electrolyte, and proceeds during the charging and discharging of the battery. Therefore, as described above, the electrode active substance plays a central role of the secondary battery from the viewpoint of the system.
Then, in the above-described lithium ion secondary battery, a lithium-containing transient metal oxide is used as a positive electrode active substance; a carbon material is used as a negative electrode substance; and charging and discharging are carried out by using the intercalation reaction and elimination reaction of lithium ions to these electrode active substances.
As a lithium-containing transient metal oxide used as the positive electrode active substance, lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganate (LiMn2O4), and others are known from the past. Among these, LiCoO2 is widely adopted because of having good charging/discharging characteristics and energy density as compared with LiMn2O4 or the like.
However, LiCoO2 has problems of having large resource restrictions, being expensive, and containing highly toxic Co. Also, LiCoO2 releases a large amount of oxygen at a temperature around 180° C., thereby raising a problem also in view of safety in a lithium ion battery using a flammable organic electrolyte. For this reason, when LiCoO2 is used as an electrode active substance, it is suitable for a small-capacity secondary battery but has a lot of problems to be solved in using it in a high-output large-capacity secondary battery.
Thus, in recent years, as an electrode active substance for a lithium ion secondary battery, lithium iron phosphate (LiFePO4) having an olivine-type crystal structure is attracting people's attention. This LiFePO4 contains phosphorus (P) as a constituent element, and all of the oxygen is covalently bonded firmly to phosphorus. For this reason, it does not release oxygen even at a high temperature, is stable in thermal stability, and is considered to be suitable for application to an electrode active substance for a high-output large-capacity secondary battery.
Then, Non-Patent Document 1, for example, reports synthesis and electrochemical properties of LiFePO4 doped with a metal obtained from a waste slag of FeSO4.7H2O.
In this Non-Patent Document 1, first, after FeSO4.7H2O is dissolved into water, H2PO4 and H2O2 are added and stirred, thereby to prepare a mixed aqueous solution. During the process, divalent Fe is oxidized to be trivalent by oxidation function of H2O2. Thereafter, ammonia water is dropwise added to adjust the pH value to be about 2.1, whereby a precipitated powder of FePO4.nH2O is obtained.
In other words, in Non-Patent Document 1, by oxidation function of H2O2 and dropwise addition of ammonia water, a precipitated powder of an Fe compound (FePO4.nH2O) containing trivalent Fe that is stable in ambient atmosphere is obtained from an Fe compound (FeSO4.7H2O) containing divalent Fe that is unstable in ambient atmosphere and is liable to be oxidized.
Further, crystalline LiFePO4 is obtained by allowing FePO4.nH2O to react with a lithium compound to obtain amorphous LiFePO4 and thereafter performing a thermal treatment.
Non-Patent Document 1: “Synthesis and electrochemical properties of metals-doped LiFePO4 prepared from the FeSO4.7H2O waste slag” by Ling Wu et al., Journal of Power Sources, 189, 2009, p. 681-684